In recent years, there has been a rapid advancement in the development of potable and cordless electronic apparatuses for consumers. Accordingly, demand is now growing for small-size and light-weight batteries with higher energy density as a power source for driving these electric appliances. In particular, lithium ion secondary batteries are expected to grow greatly in the future as a power source for notebook computers, cell phones, AV devices, etc because they have high voltage and high energy density.
For the positive electrode for a lithium ion secondary battery, lithium-containing composite oxides such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4 are employed.
For the negative electrode, on the other hand, various kinds of carbonaceous materials are used. Although it is known that carbonaceous materials include crystalline materials and amorphous materials, crystalline graphite is mostly used these days. The reasons why graphite is mostly used for the negative electrode include: (i) capacity per weight is large, (ii) the carbon density of a negative electrode material mixture layer is increased, (iii) the initial irreversible capacity of the negative electrode is low, etc. Thus, there have been studies on a higher capacity negative electrode using graphite.
Since the theoretical capacity of graphite is 372 mAh/g, the efforts to create a higher capacity material have their limitations. In order to further reduce the irreversible capacity, methods such as to optimize the composition of a non-aqueous solvent in a non-aqueous electrolyte and the surface state of graphite have already been taken. Therefore, the only approach left to create a higher capacity negative electrode is presumably to increase the carbon density of a negative electrode material mixture layer. For example, Japanese Laid-Open Patent Publication No. 2000-195518 and Japanese Laid-Open Patent Publication No. 2000-294283 disclose that the use of a mixture of graphitized carbon fiber and graphite gives a negative electrode with a carbon density of not less than 1.6 g/cm3, which is determined by dividing the weight of the carbonaceous material in the material mixture layer by the volume of the material mixture layer.
Japanese Laid-Open Patent Publication No. 2000-195518 discloses a negative electrode comprising a mixture of carbon fiber material and another carbonaceous material, wherein acrylic rubber-based copolymer is contained as a binder. The publication also describes that, in the case where the negative electrode has a carbon density of not less than 1.3 g/cm3, the following tendencies are observed: a conductive network within the carbonaceous materials is enhanced, the electrode utilization rate is increased and the binding property between active materials is improved.
However, the problem arises that the irreversible capacity of carbon per weight is increased when the negative electrode is rolled until the material mixture layer has a carbon density of not less than 1.4 g/cm3 in order to obtain a higher capacity negative electrode comprising a mixture of graphitized carbon fiber and flake graphite. Although the details of the cause are unknown, it is presumably because graphite particles are broken into fine particles by the excessive rolling to increase the surface area of the negative electrode. The increased surface area of the negative electrode facilitates the decomposition reaction of the non-aqueous solvent contained in the battery, which occurs on the negative electrode surface. The decomposition reaction of the non-aqueous solvent increases the irreversible capacity of the negative electrode.
Furthermore, in the case where the negative electrode material mixture layer has a carbon density of not less than 1.6 g/cm3, another problem arises that the capacity characteristics are largely reduced at high rate charge/discharge. The reason why the capacity characteristics are impaired is presumably because the basal planes of the flake graphite contained in the material mixture layer are oriented in parallel with the electrode plate surface since the negative electrode is excessively rolled, rendering it difficult for lithium ions to move within the negative electrode.